1. Field of the Invention
This invention relates to a test apparatus and more particularly to an apparatus for measuring the current-voltage characteristics of a TRAPATT diode.
2. Description of the Prior Art
It is often desirable to evaluate the performance of devices such as TRAPATT diodes before they are mounted in a circuit to eliminate the time and cost of packaging and tuning such devices of poor quality. Furthermore, a TRAPATT diode, because of its relatively high operating power, will sometimes burn out when the diode itself or other elements in an oscillator or amplifier circuit malfunction. It is difficult, if not impossible, to select in advance a replacement diode without pretesting the diode in its actual circuit environment.
According to present understanding of principles of operation, a TRAPATT diode has a threshold voltage level which when met or exceeded triggers the diode into the so-called TRAPATT mode of operation. Typically, in the case of an oscillator circuit, the diode is reverse biased by a pulsed or D.C. voltage equal to or greater than the threshold value required for TRAPATT operation. For an amplifier, the diode is usually reverse biased by a pulsed or D.C. voltage somewhat below the threshold value. The diode will be triggered into the TRAPATT mode when the reverse bias voltage combines with an applied RF signal of sufficient amplitude.
When the threshold conditions for the TRAPATT mode are met, the diode operation voltage drops sharply and a large current is drawn from the bias voltage power supply giving rise to the so-called "transit-time" oscillation at a certain frequency. These "transit-time" oscillations of the TRAPATT mode create a negative resistance region in the diode's current-voltage characteristic curve. It has been found that the magnitude of the negative resistance region is related to the high power performance of TRAPATT diodes. Thus by identifying the negative resistance region of such diodes it is possible to predict the diodes' performance in a circuit.
None of the known prior art diode testing arrangements provides the conditions for testing TRAPATT diode operation outside of its intended circuit environment, particularly the measurement of the negative resistance region of the TRAPATT mode. Prior art arrangements for testing TRAPATT diode operation may be termed in situ testing. The shortcoming of such a test procedure is mentioned above: the failure of a diode requires reconstruction of the circuit. Testing circuits conventionally used for "low current" diodes will not properly test the performance of a TRAPATT diode. One of the problems encountered is that such conventional test circuits often oscillate during the TRAPATT mode of operation. As a result of this problem, diodes are often packaged in their intended circuits in order to evaluate them properly as an in situ test.